Merge patch series "Switch to using $(PHASE_) in Makefiles"

Tom Rini <trini@konsulko.com> says:

This series switches to always using $(PHASE_) in Makefiles when
building rather than $(PHASE_) or $(XPL_). It also starts on documenting
this part of the build, but as a follow-up we need to rename
doc/develop/spl.rst and expand on explaining things a bit.

Link: https://lore.kernel.org/r/20250401225851.1125678-1-trini@konsulko.com

doc/develop/codingstyle.rst

@@ -154,6 +154,73 @@ See `here
 <https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#function-documentation>`_
 for style.
 
+Conditional Compilation
+-----------------------
+
+Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
+files; doing so makes code harder to read and logic harder to follow.  Instead,
+use such conditionals in a header file defining functions for use in those .c
+files, providing no-op stub versions in the #else case, and then call those
+functions unconditionally from .c files.  The compiler will avoid generating
+any code for the stub calls, producing identical results, but the logic will
+remain easy to follow.
+
+Prefer to compile out entire functions, rather than portions of functions or
+portions of expressions.  Rather than putting an ifdef in an expression, factor
+out part or all of the expression into a separate helper function and apply the
+conditional to that function.
+
+If you have a function or variable which may potentially go unused in a
+particular configuration, and the compiler would warn about its definition
+going unused, mark the definition as __maybe_unused rather than wrapping it in
+a preprocessor conditional.  (However, if a function or variable *always* goes
+unused, delete it.)
+
+Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
+symbol into a C boolean expression, and use it in a normal C conditional:
+
+.. code-block:: c
+
+	if (IS_ENABLED(CONFIG_SOMETHING)) {
+		...
+	}
+
+The compiler will constant-fold the conditional away, and include or exclude
+the block of code just as with an #ifdef, so this will not add any runtime
+overhead.  However, this approach still allows the C compiler to see the code
+inside the block, and check it for correctness (syntax, types, symbol
+references, etc).  Thus, you still have to use an #ifdef if the code inside the
+block references symbols that will not exist if the condition is not met.
+
+When working with xPL (see :doc:`spl` for more information) we need to take
+further care to use the right macro. In the case where a symbol may be
+referenced with an xPL-specific Kconfig symbol, use the CONFIG_IS_ENABLED macro
+instead, in a similar manner:
+
+.. code-block:: c
+
+	if (CONIG_IS_ENABLED(SOMETHING)) {
+		...
+	}
+
+When dealing with a Kconfig symbol that has both a normal name and one or more
+xPL-prefixed names, the Makefile needs special consideration as well. The
+PHASE\_ macro helps us in this situation thusly:
+
+.. code-block:: make
+
+        obj-$(CONFIG_$(PHASE_)SOMETHING) += something.o
+
+At the end of any non-trivial #if or #ifdef block (more than a few lines),
+place a comment after the #endif on the same line, noting the conditional
+expression used.  For instance:
+
+.. code-block:: c
+
+	#ifdef CONFIG_SOMETHING
+	...
+	#endif /* CONFIG_SOMETHING */
+
 Driver model
 ------------
 

doc/develop/tests_writing.rst

@@ -309,15 +309,15 @@ an #ifdef since the suite will automatically be compiled out in that case.
 Finally, add the test to the build by adding to the Makefile in the same
 directory::
 
-  obj-$(CONFIG_$(XPL_)CMDLINE) += wibble.o
+  obj-$(CONFIG_$(PHASE_)CMDLINE) += wibble.o
 
 Note that CMDLINE is never enabled in SPL, so this test will only be present in
 U-Boot proper. See below for how to do SPL tests.
 
 You can add an extra Kconfig check if needed::
 
-  ifneq ($(CONFIG_$(XPL_)WIBBLE),)
-  obj-$(CONFIG_$(XPL_)CMDLINE) += wibble.o
+  ifneq ($(CONFIG_$(PHASE_)WIBBLE),)
+  obj-$(CONFIG_$(PHASE_)CMDLINE) += wibble.o
   endif
 
 Each suite can have an optional init and uninit function. These are run before